The Oath of Allegiance is a promise to be loyal to the British monarch, and his or her heirs and successors, sworn by certain public servants in the United Kingdom, and also by newly naturalised subjects in citizenship ceremonies. The current standard wording of the oath of allegiance is set out in the Promissory Oaths Act 1868.

Traditionally an oath is either a statement of fact or a promise taken by a sacrality as a sign of verity. A common legal substitute for those who conscientiously object to making sacred oaths is to give an affirmation instead. Nowadays, even when there is no notion of sanctity involved, certain promises said out loud in ceremonial or juridical purpose are referred to as oaths. "To swear" is a verb used to describe the taking of an oath, to making a solemn vow.